The present invention relates to marine seismic exploration and particularly to marine exploration wherein a long seismic streamer is towed behind a survey vessel. In this system a source of seismic impulses is periodically activated to produce seismic waves that travel through the formations underlying the body of water and reflected from various interfaces back to the water where they are detected by the hydrophones placed in the seismic streamer. In recent years with the development of more sophisticated methods for processing seismic data, it has become desirable to know the precise location of the seismic streamer towed behind the survey vessel. These sophisticated methods include obtaining a large amount of data and then adding a large number of individual seismic signals to amplify the signals of interest while cancelling the noise or unwanted signals. For these techniques to be successful, it is necessary to know the precise position of the hydrophone in the seismic streamer at the time the seismic data was recorded.
U.S. Pat. No. 4,068,208 describes a system for determining the orientation of a seismic streamer using a single magnetic compass located midway along the length of the streamer which, along with various instrumentation placed on the survey vessel for determining the yaw, pitch and azimuth of the streamer. While the system provides accurate information regarding the portion of the streamer adjacent the survey vessel, it provides insufficient information regarding the remainder of the streamer. The seismic streamers presently used are two to three miles in length and thus subject to current and wave action. The current and wave action can cause a portion of the seismic streamer between the tow vessel and the end of the streamer to be displaced in a completely opposite direction from the original direction of the streamer adjacent to the survey vessel.
Also, gimbaled magnetic compasses on the market today that have a resolution accuracy suitable for this application are of a size too large to fit into the normal streamer sections. Thus, special, oversized streamer sections must be manufactured and inserted between normal, active sections of the streamer cable if compasses with high resolution are to be placed inside the streamer jacket. These oversized streamer sections inserted into the cable not only destroy the overall cable geometry, but also can add noise to the streamer because of the turbulence set up by the oversized sections being towed through the water. Both of these features are deemed undesirable for seismic data acquisition.
In addition to the above problems, the system described in the above patent is also extremely difficult to service. Since the compass is mounted in the oversized seismic streamer, it necessitates disassembly of the streamer to service any of the various components. Most seismic streamers are oil-filled to maintain buoyance and provide accurate response to the hydrophones to seismic waves, thus the oil must be drained and replaced when servicing the components.